In this image taken from video obtained from the Ugarit News, which has been authenticated based on its contents and other AP reporting, Syrian rebels capture a helicopter air base near the capital Damascus after fierce fighting in Syria, on Sunday, Nov. 25, 2012. The takeover claim showed how rebels are advancing in the area of the capital, though they are badly outgunned by Assad's forces, making inroads where Assad's power was once unchallenged. Rebels have also been able to fire mortar rounds into Damascus recently. (AP Photo/Ugarit News via AP video)

Photo: Uncredited, Associated Press

In this image taken from video obtained from the Ugarit News, which...

Syrian rebels are making significant advances in their battle against government forces, raising new questions about President Bashar Assad's ability to hold onto power and adding urgency to the quest by the international community for a unified and effective political opposition that could take control should his government collapse.

In the past week, the rebels have seized five important military facilities in the north, the east and near the capital, Damascus. The rebels captured sizable quantities of weaponry, further isolating remaining government positions and freeing up rebel forces to concentrate on attacking them.

The rebels, most of them grouped loosely under the umbrella of the Free Syrian Army, have not yet demonstrated the capacity to capture any of the country's major cities, and whether they will ever be in a position to dislodge the government from the heavily guarded capital without help is in question.

Taken together, however, the gains underscore both the steadily growing effectiveness of the rebel force and the accelerating erosion of what had once been one of the region's most powerful armies, now severely depleted and on the defensive along almost all of the country's many battlefronts.

No longer is it possible to describe the war in Syria as a stalemate, said Jeffrey White, a defense fellow at the Washington Institute for Near East Policy, and the pace of rebel gains in recent weeks raises the prospect that a collapse of government forces could come sooner than has been expected.

Putting a timeline on the government's possible demise is impossible, analysts say, in part because so many other variables are in play, from the state of the Syrian economy to the prospect that the government could use even greater force, including its arsenal of chemical weapons.

In a video released overnight Saturday of the assault on the most recent base to fall, the Marj al-Sultan air base outside Damascus, fighters are seen using a tank to storm the perimeter. They claim to have destroyed eight aircraft, including three helicopters and two MiG fighter jets, contributing to the slow depletion of the air force on which the government has increasingly relied to remain in the fight in areas from which its forces have been ejected.

On Sunday, a warplane fired cluster bombs into a playground near the Marj al-Sultan base, killing at least eight children and injuring many more, activists in Damascus said.